Surge protectors are used generally for protecting electronic equipment during undesirable high voltage surges such as occur when a circuit coupled to an electronic device is struck by lightning. The surge protector is generally placed in parallel with the equipment to be protected. Under normal ambient conditions the surge protector presents a high impedance and thus has essentially no effect on the electronic equipment. However, under high voltage surge conditions the surge protector becomes a low impedance to shunt the high current surges around the electronic equipment being protected.
Surge protectors are commonly used in telephone equipment, especially telephone equipment connected to above ground telephone lines. The surge protectors in this equipment are used principally to protect the equipment from the type of surges which occur when the telephone lines are struck by lightning. Surges induced on the telephone lines by lightning typically are of relatively high power and of relatively short duration. For example, the instantaneous power surge levels received by equipment connected to the telephone lines can extend into kilowatt levels with durations of peak power of less than a millisecond.
In the past, the generation of controlled power surges for testing surge protectors has been a problem due to the limitations of testing equipment. As a result, testing apparatus which test at less than full power or stress levels often have been used.
For example, U.S. Pat. No. 4,075,549 to Roger P. Woodward discloses a method of testing in which the voltage across a surge protector is continuously increased while the current through the surge protector is monitored. When the current through the surge protector increases rapidly, i.e., when the impedance of the surge protector decreases rapidly, the voltage across the surge protector at the time when the current surge occurs (the breakdown voltage) is recorded, and the voltage across the surge protector is immediately reduced by the tester.
While this method produces a threshold voltage measurement, such a test set does not simulate the actual conditions which a lightning surge places onto a surge protector. As a result some surge protectors fail in actual use due to the severe electrical stress placed on them. Fortunately, the vast majority of surge protectors fail by shorting permanently rather than by opening. The equipment is thus protected, but is rendered inoperable until the shorted surge protector is replaced.
In a telephone network, many lines can receive a voltage surge from a single bolt of lightning, thus exposing many surge protectors to a lightning surge. It can be readily appreciated that the cost of replacing defective surge protectors can be considerable. Thus a testing apparatus which produces a lightning simulation surge to eliminate devices which would fail in use is very desirable.
Another previously used testing method which more accurately presents a lightning surge type of test condition to a surge protector involves the use of a series RC circuit. In this type of test apparatus a large capacitor is charged with a voltage greater than the specified breakdown voltage of the surge protector under test. A switch is then closed, placing the surge protector and a resistor in series with the capacitor. The capacitor and resistor are chosen such that the initial current through a nominal surge protector typically is on the order of up to 100 amps. The current immediately starts to decay from the initial magnitude at an exponential rate, and the voltage across the surge protector is measured approximately 10 microseconds after the switch is closed. This type of testing is advantageous in that it provides a high peak power, short duration pulse as would be encountered by a surge protector protecting electronic equipment struck by lightning. The disadvantage with this test method is that the surge protector breakdown voltages can vary greatly from device to device, as much as 100% over nominal, and as little as a short circuit, i.e., zero. Thus, for a given series resistor the current through the surge protector often varies greatly and directly affects the accuracy of the breakdown voltage test and the stress induced in the surge protector.
Therefore, it can be appreciated that a surge protector test apparatus and method which accurately tests the characteristics of surge protectors under conditions similar to those produced by high voltage surges such as lightning is highly desirable.